sage.schemes.elliptic_curves.isogeny_class: mark one test "long time"

Ine test in this file is causing a "slow doctest" warning on the
CI. On my (much slower) machine, it takes over 40s:

  File "src/sage/schemes/elliptic_curves/isogeny_class.py", line 1412,
  in sage.schemes.elliptic_curves.isogeny_class.?
    Warning: slow doctest:
      [phi.degree() for phi in E3.isogenies_prime_degree()]
  Test ran for 43.57s cpu, 43.56s wall
  Check ran for 0.00s cpu, 0.00s wall

We mark it "long time" to avoid the warnings (and because it takes a
long time).

Author: orlitzky

src/sage/schemes/elliptic_curves/isogeny_class.py

@@ -1406,8 +1406,9 @@ def possible_isogeny_degrees(E, algorithm='Billerey', max_l=None,
 
     Over an extension field::
 
+        sage: # long time
         sage: E3 = E.change_ring(CyclotomicField(3))
-        sage: possible_isogeny_degrees(E3)                                              # long time (5s)
+        sage: possible_isogeny_degrees(E3)
         [5]
         sage: [phi.degree() for phi in E3.isogenies_prime_degree()]
         [5, 5]